Documents released by the National Archives could be key factor in quashing 40-year-old convictions

Des Warren (left) and Ricky Tomlinson, the Shrewbury Two, pictured after the release of Warren in 1975. Tomlinson hopes recently released documents could lead to the conviction being quashed Photograph: PA

A letter obtained under the Freedom of Information Act shows that there was discussion at the highest level of the Heath government over the decision to prosecute trade unionists, in one of the most controversial cases of the last four decades.

Surviving members of the so-called Shrewsbury 24, including actor Ricky Tomlinson, believe that a letter from the then attorney general, Sir Peter Rawlinson, to the then home secretary, Robert Carr, could be a key factor in having their 1973 convictions quashed.

The letter was unearthed from the National Archives at Kew under the 30-year rule by researcher, Eileen Turnbull. It now forms part of the evidence assembled for the Criminal Cases Review Commission (CCRC) in a bid to overturn the convictions of union pickets arrested during the first-ever national building workers' strike in 1972. The strike lasted twelve weeks and resulted in a pay rise but the union's picketing tactics enraged the construction industry and the government.

Five months after the strike ended, the 24 were arrested and charged with offences ranging from conspiracy to intimidate to affray. They were convicted at Shrewsbury crown court in 1973 and six were jailed, with Tomlinson (two years) and Des Warren, (three years) receiving heavy sentences. Their case became a cause celebre for the left and the union movement.

In his letter, dated 25 January 1973, Sir Peter tells Carr that the strike had produced "instances of intimidation of varying degrees of seriousness" and he had to decide whether they should be prosecuted.

But he added that "the intimidation consisted of threatening words and ... there was no evidence against any particular person of violence or damage to property."

Sir Peter said that Treasury counsel, to whom the director of public prosecutions had referred the cases, "took the view that the prospects were very uncertain, and in the result I agreed with him that proceedings should not be instituted." He added that the accused would have the right to a jury trial which "past experience shows" the defendants would certainly choose.

"One has therefore to consider the prospects of conviction by a jury rather than by a magistrates's court and you will appreciate that accordingly different considerations apply," he concluded. "First, the delay involved in bringing the case to trial would lend an air of unreality about the proceedings long after the strike has been settled and this would be likely to work in favour of the accused. Secondly, juries tend to treat mere words more leniently than actual violence.

Thirdly, a jury would be likely to be influenced by the political factor that conviction might revive a strike atmosphere."

Despite this assessment, three weeks later the 24 were charged. The Conservative government at the time had close links with the building industry and was always suspected of being under pressure from that quarter to act.

Turnbull said that the letter and other documents clearly show that the decision to prosecute was taken for political reasons. "Its significance is that the highest law officers in the land ... were of the view that they did not have the evidence to prosecute."

She said that other relevant documents were being held back under section 23 of the Freedom of Information Act, which applies to national security. "This was a building workers strike," she said. "It was not, and could never be described as a threat to national security. We believe that the missing documents will show by whom and why the decision was taken to prosecute the pickets."

Tomlinson, now a successful actor in films and television such as The Royle Family and Brookside, said: "we have maintained our innocence for forty years. We have always known that there was political interference by Ted Heath's Conservative government in respect of the charges brought against us." He added that a lack of evidence clearly did not matter: "they decided to throw the conspiracy charge against us which was a way of getting around the evidence problem."

A spokesman for the CCRC confirmed that the case, which is being handled by the solicitors, Bindmans, was under active review. Sir Peter died in 2006, Robert (later Lord) Carr earlier this year and Des Warren in 2004.